> Hi,
> I agree with Norman. As long as control system is of concern, it is much
> better to use normalized units.

Control law block diagrams I have seen take stick input in pounds force (pilot 
inputs) and
output in degrees to actuators. I've never seen one that output control 
commands to an
aerosurface actuator in a range from 0 to 1. Have you?

>  > surface deflections in degrees, and for good reason: it's natural, it's
>  > physical. From the point of view of JSBSim, "normalized" aerosurface
> Degrees are not natural, nor physical. We may argude that *radians*
> might be natural, but *not* degrees.

By "natural" I mean that it's: the most commonly seen angular command unit for
aerosurfaces, that it's what is used by the rendering routines to rotate 3D 
objects, that
it completely specifies the commanded angular position without the need for a 
range (a
range of 0 to 1 by itself specifies nothing without the definition of what the 
is - there is no standard here for that), and much aero data is 
non-dimensionalised using
degrees (or radians, see below). So, sorry, but based on the above description, 
for this
application, yes, degrees are "natural".

> This would lead us to another class of problems, what system of
> measurements is used? (I'm used to SI system) or
> what about input (I mean stick, pedals positions...)?
> Should the input be expressed in "natural" or normalized units?

I've said several times that we expect INPUTS in a range from zero to 1. We can 
the inputs to arrive at a force unit to match the FCS block diagram. Note that 
for JSBSim
we are also changing the config file format. When the next major version of 
JSBSim is
released (early next year) supporting the new configuration file format, many 
items will
now take a UNIT="" attribute, allowing aircraft to be defined in different unit 

> And about FDM itself, aerodata to be used are not unified... I have seen
>   some using degrees as a control surface deflections units, and others
> using radians. What would you choose as a "natural"?

True, I've seen both. JSBSim has used both, and we accept both, but 
"normalized" units are
anything but normal - you have to provide a range for it to mean anything, and 
as far as I
can tell, there is no standard here. It's defined on a per-aircraft basis. And, 
as I have
pointed out above, for aerosurfaces it requires an intermediate conversion 


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